This invention relates to fluid control valve apparatus equipped with logic valve means operative in response to pilot pressure signals, and, more particularly, to a fluid control valve apparatus of the type described which has particular utility in controlling a supply of hydraulic fluid to a hydraulic actuator in a hydraulic fluid circuit for a hydraulic construction machine, such as, for example, a hydraulic excavator.
In prior art hydraulic construction machines, such as hydraulic excavators, been usual practice to use fluid control valve apparatus of the type including spool type directional control valves is used for controlling a supply of hydraulic fluid to the hydraulic actuator. A disadvantage of this type of directional control valves resides in the fact that, since a sliding gap is necessary between the spool valve body and the valve casing, internal leaks of the fluid through the gap is unavoidable and, when the actuator is a cylinder, an inadvertent gravitational displacement of the cylinder by might occur.
Recently, proposals have been made to use poppet type logic valves to constitute fluid control valve apparatus with an eye to the advantage offered by them that internal leaks of the fluid can be eliminated.
Generally, the proposed poppet type logic valves each comprise a poppet type valve body located for movement in a valve chamber defined in a valve housing, and a conical surface portion is formed on a forward end of the valve body. The conical surface portion is brought into fluidtight contact with a valve seat formed in the valve housing when the valve body moves in a valve closing direction, to bring first and second hydraulic fluid chambers out of communication with each other. This brings a first main port formed in the first hydraulic fluid chamber out of communication with a second main port formed in the second hydraulic fluid chamber. A pilot chamber is defined between an end of the valve body opposite to the conical surface portion and the inner surface of an end cover of the valve housing, and a pilot pressure signal is applied through a pilot port formed in the end cover to the pilot chamber which has a spring mounted therein, so that the valve body is urged by the pilot pressure signal and the biasing force of the spring to move to a position in which the valve body closes the logic valve.
When the pilot pressure signal is at a reservoir pressure level (zero or substantially zero), a flow of pressurized fluid from the first main port to the second main port acts on the pressure receiving area of the valve body which corresponds to the diameter of the valve body and forces same to move upwardly to bring the first and second main ports into communication with each other. A flow of pressurized fluid from the second main port to the first main port acts on the annular pressure receiving area of the valve body which corresponds to the difference between the diameter of the valve body and the diameter of the valve seat and forces same to move upwardly to bring the first and second main ports into communication with each other.
When the pilot pressure valve reaches a predetermined high pressure level or the pilot port is blocked, the valve body is forced to move upwardly by the pressure at the first and second main ports is interrupted, thereby bringing the first and second main ports out of communication with each other.
When the fluid control valve apparatus equipped with logic valves of the aforesaid construction is used in a hydraulic fluid circuit including a hydraulic actuator for controlling a supply of hydraulic fluid to the hydraulic actuator, the first and second main ports are connected to main circuit lines thereof, and the pilot port is connected through a pilot valve to shuttle valves connected to the main circuit lines and a reservoir, so that the pilot port is able to selectively introduce into the pilot chamber as a pilot pressure signal either a highest pressure in the hydraulic fluid circuit or a reservoir pressure. When the hydraulic actuator is connected to a hydraulic pump in an open circuit, the logic valves used are four in number and a fluid control valve apparatus comprising four logic valves is conventional.
In the fluid control valve apparatus of the aforesaid construction, when the logic valve is to be opened, the pilot valve is actuated to introduce a reservoir pressure into the pilot port as a pilot pressure signal. This causes the main circuit line pressure in the first main port or second main port to force the valve body to move upwardly to bring the valve to an open position. When the logic valve is to be closed, the pilot valve is actuated to introduce into the pilot port as a pilot pressure signal a highest pressure in the hydraulic pressure circuit. As a result, a force tending to urge the valve body to move downwardly by the pressure in the pilot chamber overcomes a force tending to move the valve body upwardly by the main circuit line pressure in the first main port or second main port, to thereby bring the valve to a closed position.
As described hereinabove, in fluid control valve apparatus of the prior art, the logic valve is opened or closed depending on the relative magnitudes of the force tending to force the valve body to move upwardly by a pilot pressure signal and the force tending to force the valve body to move downwardly by a pressure in the first main port or second main port. Thus, the logic valve is of a construction such that the operation is essentially susceptible to the influences exerted by a variation in the pressure in the first main port or second main port. Also, in the fluid control valve apparatus in the aforesaid specific connection, the first main port or second main port of the logic valves has a main circuit line pressure of the hydraulic fluid circuit applied thereto which has a value tending to undergo a variation of high magnitude depending on the operating condition of the hydraulic actuator. Thus, the operation of the logic valves is influenced by a variation in main circuit line pressure. For example, assume that the main circuit line pressure introduced into the first and second main ports reaches a highest level of the pressure prevailing in the hydraulic fluid circuit and the highest pressure is introduced as a pilot pressure signal into the pilot port which has been brought out of communication with the reservoir when the valve is to be closed. Then the speed at which the valve is closed would become extremely low, showing poor responsiveness, since the valve body is operated only by being downwardly moved by the spring installed in the pilot chamber. Also, when the reservoir pressure is introduced into the pilot port as a pilot pressure signal to open the valve where the main circuit line pressure introduced into the main port is extremely high or when the highest circuit pressure is introduced into the pilot port as a pilot pressure signal to close the valve where the main circuit line pressure introduced into the main port is extremely low, the difference in pressure between the pressure in the main port and the pressure introduced into the pilot port as a pilot pressure signal becomes great, so that the speed at which the valve is opened or closed would become extremely high and the flow rate of the hydraulic fluid would show a sudden change, thereby giving a great shock to the hydraulic fluid circuit and the actuator. More particularly, when the speed at which the valve is closed becomes extremely high, the valve body is thrown against the valve body, so that the conical surface portion of the valve body is damaged by the valve seat. Also, abrupt blocking of communication between the first main port and second main port applies a surge pressure to the first or second main port due to the water hammer phenomenon, thereby exerting injurious influences on the hydraulic fluid circuit or actuator.
The fluid control valve apparatus of this construction has suffered an additional disadvantage in that since a high pressure is introduced from the hydraulic fluid circuit to the pilot port of the logic valves, the high pressure would also be applied to a pilot line and the latter would tend to suffer damage.